1. Field of the invention
This invention relates to a light exposure device for a master optical disc used as a master disc in manufacturing an optical disc formed with pits and grooves for exposing the master optical disc to light.
2. Related Art
In general, a recordable optical disc is formed with concentric grooves or a spirally extending groove. There is an optical disc of the groove-recording type in which information signals are recorded in the grooved portion. There is also an optical disc of the land-recording type in which information signals are recorded on lands which are protuberant portions formed between neighboring turns of the groove(s). There is further an optical disc of the land-and-groove recording type in which the information signals are recorded in the groove(s) and the lands.
In any of these systems, a master optical disc is prepared in the so-called mastering process, and the optical discs are prepared from the master disc, a father disc, a mother disc and a stamper. That is, in producing an optical disc, a master optical disc having crests and valleys corresponding to the groove(s) and lands formed to high precision is required. In the following description, the portions formed in the master optical disc in register with pits of the optical disc are termed "pit-forming portions", while the portions similarly formed in the master optical disc in register with groove(s) of the optical disc are termed "groove-forming portions".
When preparing the master optical disc, a laser light beam is illuminated on a photoresist coated to a pre-set thickness on a glass substrate for forming the pit-forming and groove-forming portions by light exposure. That is, with the master optical disc, the laser light beam is illuminated on the pit-forming and groove-forming portions, and the areas of the photoresist illuminated by the laser light beam are etched during the development process, while the areas thereof other than those illuminated by the laser light beam are cured. This forms the pit-forming portions and the groove-forming portions having the desired shape on the master optical disc.
In illuminating the laser light beam on the photoresist, the wobbling method, as described in Japanese Laying-Open Patent Publication 64-35742, is used. With the wobbling method, the laser light beam illuminated on the photoresist is oscillated along the radius of the master optical disc for forming pits and groove(s) to wider widths.
For effectuating light exposure using this wobbling method, a light exposure device 101 having a pair of wedge-shaped cylindrical lenses 100A, 100B, as shown in FIG. 1, is used. The light exposure device 101 includes a light source, not shown, for radiating a laser light beam, the paired cylindrical lenses 100A, 100B, arranged on the optical axis of the laser light beam radiated from the light source, an acoustic optical element 102, arranged between the paired wedge-shaped cylindrical lenses 100A, 100B, a light condenser lens 103 arranged on the optical axis of the laser light transmitted through the for condensing the laser light wedge-shaped cylindrical lenses 100A, 100B and an objective lens 104 arranged downstream of the focal point position of the light condenser lens 103.
In the light exposure device 101, the laser light transmitted through the paired wedge-shaped cylindrical lenses 100A, 100B has its pre-set direction condensed, while having its direction perpendicular thereto collimated so as to enter the acoustic optical element 102. That is, the laser light entering the acoustic optical element 102 is in the form substantially of a spot having the direction of light condensation as the longitudinal direction.
The acoustic optical element 102 wobbles the laser light entered thereto in a state in which compression waves of preset frequencies are superimposed thereon. The compression waves are superimposed from the direction parallel to the longitudinal direction of the substantially linear spot shape for wobbling the laser light.
The laser light beam wobbled by the acoustic optical element 102 is restored to its original shape, as it is transmitted through the opposite side wedge-shaped cylindrical lens 100B, so as to be illuminated on the condenser lens 103. The condenser lens 103, having its focal point position upstream of the objective lens 104, condenses the wobbled laser light on its focal point position. The wobbled laser light is exposed to the master optical disc 105 via the objective lens 104.
When the master optical disc is exposed to light by the wobbling method, a light exposure device 110 is sometime used, as shown in FIG. 2. This light exposure device 110 includes a light source, not shown, a light condenser lens 111 arranged on the optical axis of the laser light outgoing from the light source, an acoustic optical element 112 arranged upstream of the light condenser lens 111 and an objective lens 113 arranged downstream of the focal point of the light condenser lens 111.
With the light exposure device 110, the laser light outgoing from the light source is condensed by the light condenser lens 111 so as to enter the acoustic optical element 112 at a point ahead of the focal point position. Similarly to the above-described acoustic optical element 102 of the light exposure device 101, the acoustic optical element 112 superimposes the compression waves on the laser light. The laser light entering the acoustic optical element 112 of the light exposure device 110 has its profile changed to a circular shape. The wobbled laser light is exposed via objective lens 113 on a master optical disc 114.
Meanwhile, for manufacturing a master optical disc, having crests and valleys formed to high precision by the wobbling method, it may be contemplated to use the compression wave of high frequency in the above-described acoustic optical elements 102, 112. That is, by using high-frequency compression waves, the oscillations of the wobbled laser light can be increased to enable the edges of the light-exposed area to be formed to high precision. For forming pits or grooves of broader widths by the wobbling method, it may be contemplated to increase the amplitude of the wobbled laser light.
In the light exposure device 101, shown in FIG. 1, the acoustic optical element 102 superimposes the compression wave on the laser light, condensed to a substantially linear light beam, from the direction parallel to the longitudinal direction of the spot shape. If, in the present light exposure device, the high frequency compression wave is used, the longitudinal width of the spot shape is substantially equal to the wavelength of the compression wave. This renders the direction of diffraction of the laser light in the light flux to be non-uniform to cause scattering of the laser light. Thus, in this case, the wobbled laser light exposing the master optical disc to light is not of the desired spot shape.
Consequently, the master optical disc is exposed to light in areas other than desired areas, so that the disc cannot have the pit-forming portion or the groove-forming portion of desired shape. That is, the above-described light exposure device has a drawback that it is difficult to perform light exposure so that the pit-forming portions and the groove-forming portions will present the shape of recesses and crests of high precision.
On the other hand, in the above-described light exposure device 110, the acoustic optical element 112 is arranged upstream of the focal point position of the light condenser lens 111. Also, in this case, since the acoustic optical element 112 uses compression waves of high frequency, the laser light of a small spot diameter needs to be supplied.
In this case, the acoustic optical element 112 needs to be arranged in the vicinity of the focal point position of the light condenser lens 111. The light deflection by the acoustic optical element 112 becomes smaller the closer the acoustic optical element 112 is arranged towards the focal point position of the light condenser lens 111 in association with the compression waves of higher frequency. That is, in the present light exposure device 110, the laser light is of a small amplitude.
Thus, the above-described light exposure device 110 has a drawback that the master optical disc cannot be exposed to light in register with the groove-forming portion or the pit-forming portion adapted for forming larger wobbling pits.